ABSTRACT Dramatic advances in surgical repair and cardiac intervention have improved survival in even the most complex forms of congenital heart disease (CHD). With this notable success, there has been a shift from perioperative to chronic cardiac morbidity and accelerated mortality. Right ventricular (RV) dysfunction is an important determinant of long-term outcomes in children and adults with many forms of CHD. Outcomes such as RV dysfunction and associated comorbidities are currently thought of primarily in terms of hemodynamic or physiological factors. However, routine clinical and imaging variables have explained only a small percentage of the variability in RV function and clinical outcomes in CHD patients, suggesting an important role for as-yet- unrecognized contributors. We hypothesize that multiple genetic factors contribute to the unexplained variation in RV performance and patient outcomes. To investigate the relationship of genomic factors and clinical outcomes, we will study two exemplars of CHD for which right ventricular (RV) dysfunction especially impacts outcomes: tetralogy of Fallot (TOF) and hypoplastic left heart syndrome (HLHS). Our proposed study population will leverage a unique clinical and genetic database developed by the PCGC, as well cohorts within individual PCGC centers and other consortia. In Aim 1, we will study the effects of rare damaging variants identified in patients with TOF and HLHS on RV function, clinical outcomes, and anatomical subtypes influencing outcomes. Our primary outcome will be RV ejection fraction by cardiac MRI (CMR). Secondary outcomes will include other CMR measures of systolic and diastolic function, as well as clinical outcomes such as transplant-free survival, sustained ventricular and atrial tachycardias, and heart failure defined as New York Heart Association Class III or IV. In Aim 2, we will study the effects of common variants identified in patients with TOF and HLHS on RV function, clinical outcomes, and anatomical subtypes influencing outcomes. Primary and secondary outcomes will be identical to those in Aim 1. Aim 3 will assess the effects of rare and common variants associated with outcomes on cardiomyocyte function, metabolism, gene expression, and chromatin accessibility in isogenic induced pluripotent stem cells differentiated into cardiomyocytes (iPSC-CMs) and CHD tissues. We will define outcome-associated common variants using bioinformatic and functional assays, and derive iPSC-CMs with CHD variants, alone or in addition to rare and common outcome-associated variants. We will assess contraction relaxation, energetic parameters, and transcriptional activities in iPSC-CMs. We will complement these studies with single cell nuclear sequencing (NucSeq) and ATACseq analyses of CHD tissues to explore how outcome-associated variants influence in vivo cardiomyocyte biology. By identifying genes affecting outcomes, our proposal will advance mechanistic insights, improve risk-stratification and provide resources for more precise personalized therapies for CHD patients across the lifespan.